Let’s take a very simple situation.  We can imagine a case where the Husband has moved out of the matrimonial home and is living in rented accommodation.  The Wife is at home with the children but she does have some money coming in from benefits and some part-time work.  Here’s a schedule of income and outgoings for both parties.  It is on a monthly basis:

In our example the Wife’s outgoings are higher.  This is because the mortgage on the family home is higher than the Husband’s rent on his property.  Also the Wife has most of the care of the children.

In a situation like this, it seems likely that some maintenance would need to be paid from Husband to Wife to help Wife meet her income shortfall.  Of course there are all sorts of arguments that the parties might run.  For example the Husband might be saying that the family home is larger than the Wife and the children need and that it should be sold.  The Wife could then move into a smaller property, her income need would be less and so the maintenance could be less.  He might also argue that the Wife could earn more.